This invention relates to a calcium sulfate hemihydrate composition and method for using it. Further, it relates to a universal primer that can be used on new or previously painted gypsum panels, cement panels or the like to prepare the surface for either plastering, painting or other form of surface decorative finish.
Interior walls are typically made of gypsum panels, such as SHEETROCK® brand gypsum panels, cement panels or the like. While the use of panels allows large areas of wall to be created quickly, seams between the panels must be finished to make a monolithic surface that is ready to receive paint or another decorative finish. Whether new walls are being constructed or old walls resurfaced, a great deal of time is necessary to prepare the walls for paint, plaster or other decorative finishes.
Finishing of new panels is a time consuming process. After the boards are hung, a coat of joint compound is applied to the seams and the nail heads. The joint compound is allowed to dry overnight, then is sanded when completely dry. Dust from sanding is removed with a tack cloth or damp sponge. If the joint compound shrinks, a second coat of joint compound is applied and allowed to dry overnight followed by sanding and dedusting of the surface. For a quality job, the process is repeated with a third coat of joint compound. Finishing of the surface takes at least three days, even after the boards are hung. This process can also be used to repair damaged walls.
Although this process takes time, it produces a smooth, monolithic surface at a low cost. It is a relatively simple process and can often be done by a homeowner without professional assistance. But there are drawbacks in the amount of time needed to complete such a job. In new construction, installation of cabinets, carpet, fixtures and other finishing items is held up while waiting for the finishing to be completed. Workmen have to keep returning to the same job site several days in a row.
For a homeowner doing repairs or redecorating, disruption of the home is prolonged. Repeated sanding of the surface generates fine dust that permeates closets and cabinets, requiring a thorough dusting throughout the home. A quicker and cleaner method of refinishing old walls would be desired by both contractors and homeowners.
In place of going through this long process, some homeowners choose to apply a veneer coat of plaster over the entire wall. After plastering over the seams, fasteners and other imperfections, the wall is covered with plaster by a skilled craftsman. This technique results in a stronger surface that is less likely to chip, dent, scratch or incur nail pops than finishing using a joint compound. Additionally, no sanding is necessary to smooth the imperfections of the plaster because of the densification step inherent to the plastering process. By not having to sand, the homeowner is spared the mess and time needed to clean up the dust that would be generated by using joint compound. Finally, the veneer plaster can be completed in one day, compared to two to three days for joint compound work. Thus, the job is completed more quickly. Use of colorized plaster can even eliminate the need for painting of the plastered surface. A disadvantage of this technique is that the veneer plaster must be applied by a skilled crew, driving up the cost. It would be very difficult for a typical homeowner to apply a veneer plaster coat with good results.
Wallboard is available that is designed for use with plaster, such as IMPERIAL® brand gypsum board by United States Gypsum Co., Chicago, Ill. This wallboard has properties to ensure plaster integrity and additives in the paper that allow it to maintain strength even after absorbing water from the plaster application. However, these special building panels may not be available everywhere, or the homeowner may decide to use a plaster finish after conventional building panels have been installed. In these cases it would be advantageous to have a coating or primer available that would allow application of a veneer plaster coating to a conventional building panel.
Resurfacing of old, damaged walls also requires several steps. New plaster, for example, cannot be applied directly to old plaster as it will not adhere. The surface is roughed up, as by sanding, to expose an absorbent surface or badly damaged portions may have to be removed. Next, a bonding agent is applied to bond the plaster layers together. A base coat of plaster is then applied, followed by a thin plaster coat. After drying, the finish plaster coat is applied. At least three layers of material are applied prior to the final finish plaster coating. Resurfacing of old plaster would be easier if a primer were available that would reduce the number of plaster coats and the amount of material that have to be applied during remodeling. Further, some homeowners would like to have the option to paint old walls that have previously been plastered or plaster old walls that had been painted. To allow this to happen, a primer is needed that would adhere to both paint and to plaster. The prior art has not considered primers having the ability to bond to plaster as well as other wall finishes. In U.S. Pat. No. 4,661,161, a ready-mixed, setting-type joint compounds was formulated using calcium sulfate hemihydrate as the binder. A set retarder was added to the wet paste to stop the hemihydrate from hydrating to the dihydrate form. Hydration was intended to be initiated by the addition of a set accelerator just prior to use of the joint compound. However, it was found that the set retarder degraded over time, allowing hydration to occur during the shelf-life of the product.
Thus there is a need in the art for a composition, such as a primer, that bonds to a number of different surfaces and can be used for a number of different applications.